The present invention generally relates to pulse code modulated (PCM) signal extracting circuits, and more particularly to a circuit for accurately extracting a PCM signal inserted within a composite video signal or a signal having a format based on the composite video signal.
Presently, in addition to a composite video signal recorded in a recording medium such as a rotary recording medium (so-called video disc) and a magnetic tape, a data signal indicating the information content of the video signal and/or the recorded position (or time) of the video signal is recorded at a position which does not affect the video signal content, that is, at a predetermined position within a vertical blanking period, for example, in the form of a PCM signal. The above data signal is, for example, a signal indicating a program address or chapter address of the recorded video signal, an address of a position where the information content of the video signal is recorded or the playing time up to that position from when recording (reproduction) was started, and the like. This data signal is also used in a signal in which the information signal is not a video signal, but is of a format having a synchronizing signal, based on a composite video signal recorded with a PCM signal which is an audio signal, within an interval for the video signal, between the synchronizing signals.
In an apparatus for reproducing a recording medium recorded with the above described signal, a circuit for extracting the data signal recorded at the predetermined position within the vertical blanking period is constructed to clamp a pedestal level of the horizontal synchronizing signal. Further, the data signal is sliced at a 50% level of the amplitude of the data signal (PCM signal), with respect to the clamping level, to extract the data signal. The reason for slicing the PCM signal at the above 50% level is because when the PCM signal indicating the data is reproduced, the waveform is distorted and is not reproduced in the form of a square wave. Accordingly, when the PCM signal is sliced at a level other than the 50% level which corresponds to exactly half the amplitude (peak-to-peak), the duty cycle of the signal obtained by the slicing becomes different, and an accurate data extracting operation cannot be performed.
Even when reproducing the same recording medium, the data signal may be reproduced with different peak values when reproduced by different reproducing apparatuses. Moreover, in a case where the type of the recording medium is changed, or due to some other causes, the data signal may be reproduced with a different peak value even when reproduction is performed by use of the same reproducing apparatus. In this case, since the slicing level is obtained with reference to the clamping level and is always constant in the conventional PCM signal extracting circuit, there was a disadvantage in that the slicing level does not always and relatively become 50% and an accurate signal extraction could not be performed. Furthermore, in the manufacturing process of the reproducing apparatus, an operation is required to adjust the slicing level so that the slicing level accurately becomes 50%.